1. Field of the Invention
This invention relates to a method and apparatus for coating a corrodible base metal, on one or both sides, with another metal which is molten. More particularly, the invention relates to a method and apparatus for spray metallizing a coating of metal onto one side of the base metal, using an electric arc.
2. Description of the Prior Art
A number of processes are known for the production of sheet steel which is galvanized on one side. These include such conventional methods as the continuous passage of sheet metal through a bath of molten metal, and continuous thermal spraying. Commercial galvanized sheet in which zinc is coated on both sides is not generally acceptable for use in automobile body construction because of complications arising from the adherence of zinc to the tips of welding electrodes in resistance welding processes. Again, while it is desirable to have the zinc covering on interior surfaces where its protective qualities are most needed, it is desirable to leave the exterior surfaces uncoated to promote the adherence of paint or other surface finishes.
In the case of the thermal spraying process, the material being sprayed, in this case zinc, is supplied to "pot guns" in bulk form and heated to a temperature of approximately 426.degree.-482.degree. C., and atomized with a propellant such as air and sprayed onto the surface to be coated. In another version of this process, wire or powder is fed into an oxy-fueled flame which melts it and air atomizes and propels it onto the surface to be coated. Another thermal spray process, electric arc metallizing, which utilizes two metal wires fed to an electric arc, is described in U.S. Pat. No. 3,546,415, herein incorporated by reference.
These processes all suffer in one way or another from a variety of deficiencies. For instance, the processes which utilize zinc wire or powder are much more expensive than those which use slab zinc, since there is a price differential of over 50% between the two forms of this material. When wire is used, in addition there is a loss due to overspray which can amount to as much as 30 to 40% of the material, leaving a net deposit efficiency of 60 to 65% of material which actually sticks to the metal surface. Galvanizing by hot dipping has several deficiencies. First, the iron-zinc, metallurgical bond, which is formed at the interface between the metals in the form of a crystalline surface, leaves a structure which is brittle and is undesirable for automobile parts, since it cannot be deep drawn; however, other techniques are available to reduce the brittleness of the coating. Another disadvantage of galvanizing is that large open vats are required to accommodate the customary widths of the sheet steel. A great deal of energy is wasted in maintaining the baths at 426.degree. C. or so, to keep the zinc in the molten state. The disadvantage of "pot gun" process is poor quality of coating and low deposit efficiency.
It is an object of the present invention to provide a method and apparatus for the continuous spray metallizing of sheet steel with molten zinc, which utilizes slab zinc, is economic in its heat requirements, and in which there is little waste material.